Fluorescence “Giant” Red Edge Effect
6.3 Optical absorption, emission, and electrical properties
Study of optical properties of CQDs and GQDs is very much important for their application in several optoelectronic devices like photodetectors, solar cell, photo- electrochemical cells, LEDs, etc. Most of the GQDs and CQDs exhibit two distinct
0.5 g coal powder
Refluxed with 5M HNO3 for 24 h Centrifuged (2770 g) for 30 min
Supernatant
SupernatantI SupernatantII
Deposit (1)
(2) (3)
(1) (2) (3) (1). Dried by Vacuum drying
(3). Centrifuged (2770 g) for 30 min (2). Redispersed in 50 mL H2O and neutralized with 0.1 M NH4OH
DepositI DepositII
(mainly metallic
hydroxide) Dried Dried (mainly
silicate)
CoalA CoalB
50 nm 50 nm
0 0.5 1
0 0.4 0.8
Height / nm
width / μm
0.0 0.2 0.4 0.6 0.8 1.0
μm
(A) (B)
(C)
Figure 6.2 (A) Treatment procedures of coal samples. (B) TEM image. (C) AFM image of GQDs.TEM, transmission electron micrograph;AFM, atomic force micrograph;GQDs, graphene quantum dots.
Reproduced with permission from Y. Dong, J. Lin, Y. Chen, F. Fu, Y. Chi, and G. Chen, Graphene quantum dots, graphene oxide, carbon quantum dots and graphite nanocrystals in coals, Nanoscale 6 (2014) 7410 7415. Copyright 2014, Royal Society of Chemistry.
Figure 6.4 (A) AFM micrograph of NGQDs. Height distribution plot is presented at inset.
(B) HRTEM micrograph of NGQDs.
Reprinted with permission from T. Majumder, K. Debnath, S. Dhar, J. J. L. Hmar, and S. P.
Mondal, Nitrogen-doped graphene quantum dot-decorated ZnO nanorods for improved electrochemical solar energy conversion, Energy Technology 4 (2016) 950 958. Copyright 2016, Wiley-VCH Verlag GmbH & Co.
Figure 6.3 Schematic growth process of S, N co-doped GQDs[44].GQDs, graphene quantum dots.
Reprinted with permission from J.J.L. Hmar, T. Majumder, S. Dhar, and S.P. Mondal, Sulfur and nitrogen Co-doped graphene quantum dot decorated ZnO nanorod/polymer hybrid flexible device for photosensing applications, Thin Solid Films 612 (2016) 274 283.
Copyright 2016, Elsevier.
absorption peaks located nearly 220 240 nm and 320 360 nm, which lead to use of deep ultraviolet (DUV) photodetector. Commonly, the absorption peak at 220 250 nm is attributed to theπ π transition of C5C bonds [47,48]and the peak at 320 360 nm is ascribed to n π transition of C5O bond[10,49,50]. The photoabsorption of GQDs and CQDs can be extended up to visible region by dop- ing various elements like boron (B)[45], nitrogen (N)[23,51], sulfur (S)[44], phos- phorus (P)[52,53], chlorine (Cl) [54], potassium (K) [55], sodium (Na) [56], etc.
Co-doping of atoms into the GQD matrix also improves optical and electrical prop- erties for certain applications [44,47,57]. Fig. 6.6A shows the optical absorption spectrum of S, N co-doped GQDs synthesized by Majumder et al.[26]. Along with
CH2OH
OH OH
OH H H
H H H
O
HO
H
+
O B O O H
H
Boric acid
Glucose B-GQDs
160°C, 3 h
Hydrothermal
HOB OH
B OH
B O
HOB
OH COOH
20 nm 2 nm
0.24 nm
(A) (B)
Figure 6.5 (A) Schematic grown scheme of boron doped GQDs (BGQDs). (B) TEM and HRTEM micrographs of BGQDs.
Reproduced with permission from T.V. Tam, S.G. Kang, K.F. Babu, E.-S. Oh, S.G. Lee, and W.M. Choi, Synthesis of B doped graphene quantum dots as a metal-free electrocatalyst for the oxygen reduction reaction, Journal of Materials Chemistry A 5 (2017) 10537 10543.
Copyright 2017, Royal Society of Chemistry.
(A) (B)
Figure 6.6 (A) Absorption spectrum of SNGQDs, C5N, C5S peaks are shown at inset.
(B) Digital photograph of SNGQDs solution under incident of different wavelengths of light.
Reprinted with permission from T. Majumder, S. Dhar, P. Chakraborty, K. Debnath, S.P.
Mondal, S, N co-doped graphene quantum dots decorated C doped ZnO nanotaper photoanodes for solar cells application, Nano 14 (2019) 1950012. Copyright 2019, World Scientific Publishing.
strong UV absorption, the spectrum shows visible photoabsorption in the wave- length region 450 600 nm due to the presence of C5N and C5S bonding.
Upon excitation of UV light, SNGQDs demonstrate excellent visible emission as depicted inFig. 6.6B. Both GQDs and CQDs exhibit excellent photoluminescence (PL) behavior similar to semiconductor QDs, which is attractive for optoelectronic devices.
However, due to the presence of surface traps, they demonstrate very low quantum yield. But it can be enhanced dramatically after surface passivation[20,58 60].
The optical as well as electrical nature of GQDs/CQDs can be modified by con- trolling the shape and size during the growth process. Fig. 6.7A shows the sche- matic representation of variation of band gap of GQDs with size. As the size decreases, the energy band gap increases similar to semiconductor QDs. Chen et al.
established a relation between emission wavelength (band gap) and size of GQDs with the help of density-functional theory (DFT)[61].Fig. 6.7Brepresents the vari- ation of emission wavelength and size of GQDs using theoretical prediction.
Despite a small deviation between theory and experimental results, the trend of var- iation of band gap with size of GQDs demonstrated good agreement with experi- mental results[61]. Fig. 6.7C shows the experimental study of variation of band gap with size by Ye et al.[61].
GQDs and CQDs often exhibit excitation-dependent PL emission. Fig. 6.8A and Bshow the excitation-dependent PL emission of NGQDs and SNGQDs at various excitation wavelengths (340 560 nm). Such red-shifted emission is attributed to the QDs size variation and presence of various impurity states. Electrical properties of CQDs/GQDs are similar to graphene oxide (GO) when it contains oxygenated functional groups. Such surface functional groups and doping of various elements also influence its electrical conductivity.
Band Gap of GQD-1< GQD-2< GQD-3< GQD-4< GQD-5Bandgap Increase
GQD-1 GQD-2
GQD-3 GQD-4
GQD-5
Size Increase
GQD-1> GQD-2> GQD-3> GQD-4> GQD-5 Size of Eg1
Eg2 Eg3
Eg4 Eg5 Near IR
Red
Orange Yellow Green Blue Violet
UV
200 250 300 350 400 450 500 550 600 650 700 750 800 900 950 1000 1050
Emission wavelength (nm)
G5
G3: ovalene
G4: cir-coronene
G2: coronene G1: benzene
G7: hexa-peri-hexabenzocoronene G8
G6
850
999.5
765 678.2
572.4 492.3
450.5 399.5
235.2 0
20 40 60 80
Quantum Dots Size (nm)
2.0 2.1 2.2 2.3 2.4
Energy (eV) 0 10 20 30 40Membrane Pore Size (kD)
0.5 1.0 1.5 2.0 2.5 3.0
Size of GQDs (nm)
Size by TEM Membrane Pore
(A) (B) (C)
Figure 6.7 (A) Schematic illustration of variation of band gap of GQDs with size and morphology. (B) Emission wavelength (nm) plot as a function of diameter of GQDs using DFT method in vacuum. (C) Variation of band gap with the size of GQDs. PL emission from samples with various sizes of GQDs (inset).DFT, density-functional theory.
Reprinted with permission from R. Ye, Z. Peng, A. Metzger, J. Lin, J.K. Mann, K. Huang, et al., Bandgap engineering of coal-derived graphene quantum dots, ACS Applied Materials
& Interfaces 7 (2015) 7041 7048. M.A. Sk, A. Ananthanarayanan, L. Huang, K.H. Lim, P. Chen, Revealing the tunable photoluminescence properties of graphene quantum dots, Journal of Materials Chemistry C 2 (2014) 6954 6960. Copyright 2014, Royal Society of Chemistry. Copyright 2015, American Chemical Society.